An inhalation apparatus has been developed, in which the principle of the ink jet method is used to eject fine liquid droplets of drug into an air flow channel, where air inhaled through a mouthpiece flows, to allow a user to inhale the drug (see Japanese Patent Application Laid-Open No. H08-511966). Such an inhalation apparatus provides an advantage that a predetermined amount of drug is precisely sprayed in the form of particles having a uniform diameter.
The general cross-section of the inhaler clearly specified in Japanese Patent Application Laid-Open No. H08-511966 is shown in FIG. 10. The inhaler includes an air inlet port 1 that introduces air to be taken from the outside into the body of a user with drug upon inhalation, an ejection head 3 that ejects the drug, and a mouthpiece 4 that the user holds in his/her mouth when inhaling the drug ejected from the ejection head 3 into the body. The ejection head 3 has ejection ports 5, and the drug, in a reservoir 7, is supplied to the ejection head 3. The inhaler is configured so that the drug is ejected in a direction generally parallel to the direction of an air stream, and the drug is conveyed without turbulence in the air stream toward an inhalation port.
The liquid droplets ejected from the ejection ports have an extremely small diameter suitable for deposition to respiratory organs, on the order of 3 μm to 8 μm, and are likely to be affected by the turbulence of an air stream in an air flow channel. The turbulence of the air stream in which the drug is conveyed may increase collisions between the liquid droplets, and increase the diameter of each liquid droplet of the inhaled drug as a result. Any change of the diameter of liquid droplets affects the site where the liquid droplets are deposited after inhalation. In addition, the turbulence may increase the tendency of the drug to become attached to the inner wall of the air flow channel. In the latter case, the amount of drug that is not inhaled after ejection is increased, resulting in waste. Such increase is not preferable from a hygienic standpoint.
The present inventors have studied an air stream in the case where the direction of drug ejection is generally parallel to the direction of the air stream, as in the case of the mentioned National Publication of International Patent Application No. 8-511966. For example, it is assumed that a cubic ejection head cartridge is installed in a cylindrical air flow channel. FIGS. 11A, 11B and 11C illustrate the simulation results on the flow of an air stream in such an air flow channel. FIG. 11C is a view illustrating flow patterns formed by liquid droplets ejected from ejection ports 5 on the same cross-section as in FIG. 11A. FIGS. 12A, 12B and 12C are views schematically illustrating the results, and FIG. 12C is a view of the air flow channel 4 and the ejection head cartridge 3 placed in the air flow channel 4 as seen from the direction of the inhalation port. An array of ejection ports 5 is arranged parallel to the direction of a longer diameter of the ellipse of a cross section of the air flow channel 4. The cross-section taken along the line 12A-12A of FIG. 12C corresponds to that taken along the line 11A-11A of FIG. 11B, and the cross-section taken along the line 12B-12B of FIG. 12C corresponds to that taken along the line 11B-11B of FIG. 11A.
The conditions used in the simulation were as follows. The aspiration rate through the inhalation port 6 was 30 L/min, so that it was assumed that air having momentum equal to the water droplets of 1.4 ml/minute was ejected through the ejection ports 5 (at a discharge rate of 1.2 L/min). The air flow channel 4 had a longer diameter of 25 mm, a shorter diameter of 10 mm, and an overall length of 25 mm, and a length from an ejection-port providing surface to the inhalation port of 15 mm. The ejection head 3 was adapted with a cube of 10 mm×10 mm×10 mm. The ejection-port providing surface included four inlet boundaries of 0.2 mm×6 mm, the longitudinal directions of which were the same as those of the ejection port arrays. The arrows in FIGS. 11A and 11B represent the vectors of wind speeds of the air flow at the start point of the arrow. That is, the length of an arrow represents the magnitude of the wind speed, and the direction of the arrow represents the direction of the air stream.
As the outlines of the results of FIGS. 11A and 11B are shown in FIGS. 12A and 12B, it was found that after the air stream that rises along the side surfaces of the ejection head cartridge 3 passes the outer peripheries of the ejection-port providing surface constituting the top head portion of the ejection head cartridge, the flow of the air stream is divided, and is likely to approach the ejection ports 5. Therefore, the air stream was found to be directed inward, and another air stream that was not parallel to the direction of drug ejection was generated. The flow patterns formed by the liquid droplets in the case where drug is conveyed in the above-described air stream are illustrated in FIG. 11C. It was found that the flow patterns formed by the liquid droplets are distributed in a narrow space at a high density because the air stream tends to be directed to the center, particularly around the outlet of the inhalation port 6.
If the air stream illustrated in the cross-sections in the direction parallel to the ejection port 5 arrays (FIG. 11A and FIG. 12A) is directed inward, the probability of collisions between the liquid droplets ejected from the ejection port arrays is increased, which is not preferable.